The Newark mayor has a strong lead over the other candidates running for the Senate seat vacated by late Sen. Frank Lautenberg

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Newark Mayor Cory Booker enjoys a strong lead over three other candidates in the Democratic primary for U.S. Senate in New Jersey, a new poll shows.

The Monmouth University poll released Tuesday shows Booker with the support of 49 percent of Democrats likely to vote in the Aug. 13 primary. His closest competitor, U.S. Rep. Frank Pallone, got 12 percent. U.S. Rep. Rush Holt received 8 percent of the vote while Assembly Speaker Sheila Oliver, the only woman in the race, got 3 percent.

More than a quarter of likely voters say they are undecided four weeks before the special election primary to fill the seat of Sen. Frank Lautenberg, who died in office in June. The general election to fill the seat for a year is Oct. 16.

While the survey identified factors that could tighten the race, it found no evidence that any of the trailing candidates would be able to turn things around before the primary.

“Cory Booker’s lead appears to be impregnable. There is very little in the poll that shows a path for the other candidates to overtake him,” said Patrick Murray, director of the Monmouth University Polling Institute.

For example, the poll found that the two congressmen are not pulling significant support away from each other. When Holt supporters were asked who they'd vote for if their candidate was out of the race, Pallone gained 2 percent and Booker gained 4. If Pallone was out, Holt would gain 3 percent and Booker 5, the survey showed.

The poll also showed the others candidates' attacks generally were not sticking to Booker.